NPR has learned that the National Institutes of Health plans to lift a moratorium on funding some very controversial experiments. These experiments use genetic engineering and human stem cells to create embryos that are part human, part animal. Scientists hope the so-called chimera embryos would lead to medical breakthroughs. They also raise some troubling ethical questions, and NPR health correspondent Rob Stein is in our studio to talk about this. Good morning.

ROB STEIN, BYLINE: Good morning, Renee.

MONTAGNE: What exactly do scientists what to do?

STEIN: So scientists want to use new genetic engineering techniques to make changes in the DNA in animal embryos to basically edit the letters in the animal's genes. And then they want to take human stem cells and put them inside these animal embryos in the hopes that the human stem cells, which can become any kind of cell or tissue in the body, will become integrated into the embryos and then develop into animals that are - have partially or even fully human parts in their bodies.

MONTAGNE: And what would they then do with these creations?

STEIN: So it's for medical research I mean the idea here is that if you have let's say pigs or sheep or cows that have you know partially or fully formed livers or kidneys or pancreases or even parts of their brain. The Bible Study them to learn many new things that you could use to prevent and treat human diseases like Alzheimer's disease Parkinson's diabetes. And another big hope that they have is they may actually be able to grow fully formed organs fully formed hearts liver kidneys and pancreas is in these farm animals that could be used for transplants and save thousands of people who are dying every year because there's a shortage of organs.

MONTAGNE: Well, it sounds like good goals, but there's going to be a lot of people out there that are going to find this pretty alarming, and it is controversial.

STEIN: Yes, it is controversial, obviously raises some really difficult ethical issues. And, you know, the critics say that this is dangerous 'cause it blurs the line between humans and other species and starts to raise questions about what are these creatures? Are they animals? Are they partially human? And if they are partially human, how do we treat them? And there are some very specific concerns. Like, the scientists don't really know for sure exactly where the stem cells would go, so there's a concern that they could go to their brains and the animals could end up with some semblance of human consciousness or human thinking. And then what do we do? How do we think about these creatures? Or what if they end up as human sperm and eggs and the animals end up breeding?

MONTAGNE: Well, OK, but the NIH has decided to try to fund these experiments. Give us a little history.

STEIN: So about a year ago, the NIH imposed a moratorium on funding any kind of research like this, saying they needed more time to study the issue. And today, they announced their decision. They're saying, yes, we would like to proceed with a way to try to fund this research but with very specific restrictions and new safeguards in place to prevent any of those kind of sci-fi scenarios that we talked about from actually happening. For example, they won't fund any experiments on very early embryos from primates like monkeys or chimps. That would be taboo because they're just too close to us.

Other animals like pigs, sheeps and cows, that would be OK, but any experiments will have to go through an extra layer of review by a special NIH committee, especially if there's any chance the animal's brains will be changed in any big way. And scientists would have to make sure that any animals that end up with human sperm or eggs are never allowed to breed. Now, the public, the general public, has about a month to comment on this before the NIH makes it final.

MONTAGNE: Well, we'll have to leave it at that. NPR's health correspondent Rob Stein on news that scientists will be allowed by the NIH to create chimeras.